This goes without saying really, but I'm going to say it anyways as a friendly reminder. DO NOT UPDATE TO FIRMWARE 3.1.3 IF YOU ARE JAILBROKEN! It is not safe yet to do so if you want to remained jailbroken and unlocked. This advice comes straight from the DevTeam via Twitter. If your silly enough to upgrade, I've got bad news...you can no longer restore to 3.1.2 unless you have your SHSH on file in cydia.

There may or may not be a jailbreak for 3.1.3 - the dev team will most likely wait until the major release coming for iPad and iPhone 4.0. GO TO CYDIA NOW and get your keys signed on the front page if you have not. That way you'll always at least be able to restore to 3.1.2.

While we all like updates, this one is really nothing special and doesn't make it worthy of losing jailbreak just for it. The update includes a fix for the iPhone 3GS battery level reporting (I've never noticed any problems), a fix for app that crashed on launch (Haha really? ) and a fix for app crashes while using Japanese Kana Keyboard.